When we talked a few weeks ago about insufficient government cell phone regulations, we discussed "SAR" values and the legal limits the government has set for the amount of radiation a phone can emit. But SAR values aren't always easy to pin down - so today we're going to look closely at what they are and how they can vary by phone.

What does SAR mean, anyway? "SAR" stands for "specific absorption rate" - it's a measure of how much radiation is absorbed by your body, given in Watts per kilogram (W/kg). Scientists test for SAR values using models of the human body filled with viscous fluid designed to mimic human tissue. The phone is placed next to the mold, and while the phone transmits a signal, a probe inside the human model measures the absorbed energy levels. The phone's SAR value is designated as the highest amount of radiation detected during these tests. What determines a phone's emission level? A phone's SAR value is largely determined by the design of its inner hardware and antenna. But, since the SAR measures how much radiation is absorbed by your body (heartening, huh?), the value is determined by both a) the amount of radiation emitted by the phone, and b) what body part is doing the absorbing (yes, some absorb radiation more than others).

SAR values for an individual phone can vary, based on several factors:

Network & frequency Most phones sold today can operate on several networks (e.g. 2G v. 3G) and at different frequencies (which band of the network the phone is using to send and receive signals). The same phone can emit different levels of radiation depending on which network and which frequency it's being used on at any given time.

Where you're using it Phones emit radiation when they're transmitting signals from your handset to a base station (cell tower), so the harder a phone has to work to send a signal, the more radiation it will emit. As a result, when you're using your phone far from a base station (say, in a rural area) or if there are physical obstacles in the way, the SAR value would be higher.

How you're using it A phone also uses more power to transmit a voice signal, so texting emits less radiation than talking.

Where you're holding it Originally, SAR measurements focused primarily on values when held at the head. But over time, cell phone use has changed - now we're carrying phones in our pockets and often leaving them there while talking on a headset or not using them.

This means less radiation to the brain (that's good) but more to the lower torso. SAR values vary based on the type of body tissue absorbing the radiation, and research has shown that skin and muscles absorb more radiation than fat and bones. This raises concern about exposures at the waist, particularly to reproductive organs that don't have a protective layer of bone (like a skull provides the brain) shielding them from cell phone radiation Also, as you'd instinctively assume, the farther away the phone is from you, the less radiation you'll absorb from it. Holding the phone 10 inches away can reduce radiation exposure by a factor of 400. (Another reason texting and a headset are good ideas - you'd be yelling to get heard while holding the phone that far away.)

SAR testing not perfect Scientists acknowledge that SAR testing has significant precision problems, and current research is lacking in many ways. For instance, we don't know much about SAR values when phones are in "data mode" (you know you love those iphone apps), when they're held at various orientations (say, flipped around in your pocket) or how SAR values change for people of different ages and body types.

Children may be especially at risk, because their tissue conducts more (it has more ions than that of adults). Also, their thinner skulls don't provide as much of a radiation shield - one recent study showed that a child's head could absorb twice the amount of radiation as an adult's. As a result, kids using high-emitting, perfectly legal cell phones could be exposed to radiation over the legal limit.

You can make it better More research is needed, but the U.S. currently lags behind Europe when it comes to knowing what's going on with cell phones (surprise!). Tell the Federal Communications Commission (FCC) and Food and Drug Administration (FDA) to update their cell phone regulations (which they haven't done since their introduction in the mid-1990's) by sending them an email using EWG's easy form.